This invention relates to the support and positioning of an electron gun assembly in a CRT which includes an antenna-type getter support.
Referring to FIG. 1, there is shown a partially cutaway lateral view of a prior art CRT 8. CRT 8 includes a glass envelope, or bulb, 10 including a front panel, or display screen, 11, a narrow neck portion 13, and a funnel portion 12 disposed intermediate the front panel and neck portion. Disposed on the inner surface of the CRT's front panel 11 is a phosphor screen 24. Disposed within the CRT's neck portion 13 is a multi-electrode electron gun 15 directing one or more electron beams onto the phosphor screen 24 on the inner surface of the front panel 11. The CRT's glass envelope 10 is evacuated and is sealed where the front panel 11 joins the funnel portion 12 and at the distal end of the neck portion 13 of the CRT. The electron gun 15 includes a stem 14, an end electrode termed the shield, or convergence, cup 17, and a plurality of aligned charged electrodes disposed intermediate the stem and the shield cup which are not shown in the Figure for simplicity. Disposed about in a spaced manner and engaging the shield cup 17 are a plurality of contact springs, or bulb spacers, 18 which engage a conductive graphite coating 21 on the inner surface of the CRT's glass envelope for maintaining the electron gun 15 in position within the neck portion 13 of the CRT. The electron gun's cathode(s) heater and charged electrodes are attached to and maintained in common alignment by means of a plurality of elongated glass beads 16.
The CRT's antenna-type getter assembly includes a getter element 20 and a getter support member 19 disposed on the inner surface of the CRT's glass envelope 10. The getter support 19 is in electrical contact with the electron gun's shield cup 17 and extends to an intermediate portion of the CRT's funnel portion 12 and terminates in the getter element 20. A getter flashing process is performed by inductive heating of the getter element 20 by means of a high frequency (HF) coil (not shown for simplicity) placed outside the wall of the glass envelope 10 and adjacent to the getter element. The electromagnetic field generated by the HF coil penetrates the glass envelope 10, inducing a current in the getter element 20 in accordance with the Faraday-Lenz Law for absorbing residual gases during CRT manufacture.
The metallic shield cup 17 functions as a charged electrode in electron gun 15. Shield cup 17 is maintained at the anode voltage and is coupled to an anode button 22 extending through the funnel portion 12 of the glass envelope 10 by means of the combination of a graphite coating 21 disposed on an inner wall of the glass envelope and the aforementioned contact springs 18. An outer graphite coating 23 disposed on the funnel portion 12 of the glass envelope 10 forms a capacitance with the inner graphite coating 21.
In the prior art CRT 8 shown in FIG. 1, the contact springs 18 are typically three in number and are equally spaced at 120.degree. intervals about the shield cup 17. If the electron gun 15 does not include a shield cup-mounted getter support, the center axis of electron gun 15 is easily aligned with the glass envelope's centerline 32 (shown in dotted line form). However, in a CRT incorporating a getter support member 19 attached to the electron gun's shield cup 17, the resultant force of the contact springs 18 and the getter support member on the shield cup is not zero, causing misalignment between the electron gun axis and the CRT's centerline. As a result, the electron beam(s) will not be incident on the center of the front panel 11 when the beams are undeflected giving rise to misregistration of the electron beam(s) with the phosphor elements on the front panel 11 and apertures in a shadow mask, or color selection electrode (which also is not shown), in a color CRT. This electron gun misalignment and electron beam misregistration degrades video image resolution and color purity. One prior art approach to correct this problem involves the asymmetric positioning of three contact springs about the shield cup to compensate for the force of the getter support member. Precisely positioning the contact springs asymmetrically about the shield cup is difficult, limiting the usefulness of this approach.
The present invention addresses the aforementioned limitations of the prior art by providing an electron gun support and positioning arrangement which maintains the electron gun coaxially aligned with the CRT's centerline by supporting the electron gun's shield cup with a plurality of spaced contact springs which apply a resultant force on the shield cup which cancels out the force exerted on the shield cup by a getter support member.